Magento 2.0.18 is the final 2.0.x release. After March 2018, Magento 2.0.x will no longer receive security patches, quality fixes, or documentation updates. To maintain your site's performance, security, and PCI compliance, upgrade to the latest version of Magento.

Codebase changes

Every Magento version release include a change in the codebase.
The scope of the change determines whether the MAJOR, MINOR, or PATCH number increases in the version.

Public vs private code changes

A Magento module’s codebase consists of public and private code.

Changes in public code always trigger MINOR or MAJOR version increases.

In most cases, modifications to private code will trigger PATCH version increases.
On rare occasions, if the Magento development team made significant modifications to private code, they will increase the MINOR or MAJOR version.

Third-party modules should not use or change private code.
If this is unavoidable, you must depend on the patch version of the core modules used.

API and customization points

Public code includes Public API
(indicated by the @api docblock tag) and Public Customization Points.

Modules call APIs to create new application scenarios.
Modifications that break the API will trigger an increase in a module’s MAJOR version.